IXPE first observation of a magnetar confirms the presence of an ultra-strong magnetic field and provides insight into the properties of the neutron star surface

The Imaging X-ray Polarimetry Explorer (IXPE) allows scientists to study polarization of X-rays emitted by astrophysical sources for the first time after more than 40 years. The first ever observation of X-ray polarization from a magnetar, a neutron star endowed with the most powerful magnetic fields in the universe, has made it possible to probe the physical state of the surface of the neutron star and to test the processes that occur in its magnetosphere. A team led by Roberto Taverna from the Department of Physics and Astronomy of the University of Padova used the Imaging X-ray Polarimetry Explorer (IXPE), a NASA-ASI satellite launched on December 9, 2021, to observe the magnetar 4U 0142+61, a neutron star located in the Cassiopeia constellation, about 13,000 light-years away from Earth. Neutron stars are the very dense remnant cores of massive stars — initially at least 10 times more massive than our Sun — that have exploded as supernovae at the ends of their lives. The remnants have masses comparable to the solar one and extreme magnetic fields, billions of times stronger than that of the Sun, that permeate their surface and surroundings. Among neutron stars, magnetars hold the stage: they shine brightly in the X-rays and show erratic periods of activity, with the emission of bursts and flares which can release in just 1 second an amount of energy millions of times larger than our Sun emits in 1 year. Magnetars are believed to be powered by ultra-strong magnetic fields, 100-1,000 times higher than in “normal” neutron stars. The quantum theory of radiation predicts that electromagnetic waves propagating in a strongly magnetized environment are polarized in two orthogonal directions, parallel and perpendicular to the magnetic field. The amount and direction of the observed polarization bear the imprint of the magnetic field structure and of the physical state of matter in the vicinity of the neutron star, and provide information inaccessible otherwise. IXPE allowed astrophysicists to measure the amount and the direction of polarization of the radiation collected from the magnetar 4U 0142+61, the first ever magnetar for which an X-ray polarimetric observation was performed. The source is highly polarized, and the polarization strongly depends on the photon energy. It starts at about at low energies, drops to and then rises again to at high energies. At the same time the polarization direction swings by exactly 90°. The results of the IXPE magnetar observation were published in the November 3 issue of Science. “The most exciting feature we could observe is the change in polarization direction with energy, with the polarization angle swinging by exactly degrees” says Taverna. “This is in agreement with what theoretical models predict and confirms that magnetar are indeed endowed with ultra-strong magnetic fields”. Roberto Turolla from the Department of Physics and Astronomy of the University of Padova explains: “The polarization at low energies is telling us that the magnetic field is likely so strong to turn the atmosphere around the star into a solid or a liquid, a phenomenon known as magnetic condensation”. “Radiation emitted from a magnetic condensate should be indeed mildly polarized, with polarization direction parallel to the magnetic field direction” remarks Silvia Zane of MSSL-UCL. “Due to scatterings in the star magnetosphere, photons polarized perpendicularly to the magnetic field should dominate at high energies, producing the observed 90° polarization swing”. “It is also worth to notice that considering Quantum Electrodynamics effects, which are included in our theoretical modeling, gives results compatible with the IXPE observation. Nevertheless, we are also investigating alternative models to explain the IXPE data, for which proper numerical simulations are still lacking” comments Jeremy Heyl of UBC Vancouver. “The IXPE mission uses new-generation X-ray polarization detectors developed in Italy, by the National Institute for Nuclear Physics (INFN) and the National Institute for Astrophysics (INAF), in collaboration with the Italian Space Agency (ASI), and exploits x-ray mirrors fabricated at NASA’s Marshall Space Flight Center” said Luca Baldini of University of Pisa and INFN. “Beyond the magnetar 4U 0142+61, IXPE is being used to study a wide range of extreme X-ray sources and lot of exciting results are coming in” adds Fabio Muleri of INAF-IAPS. In the same issue of Science, a second work, led by Henric Krawczynski at Washington University in St. Louis, discusses the IXPE detection of polarized light from the black hole X-ray binary Cygnus X-1. “We are extremely excited to see what the future IXPE observations of these and other kinds of sources will bring” concludes Taverna. More information are available on the Science press package at this link https://www.science.org/doi/10.1126/science.add0080